|Ask the Experts|
April 14, 2020
White Discoloration After Ultrasonic Cleaning
We are using halogen free solder paste SAC305 type "no clean" for our circuit board assemblies. We are cleaning using an ultrasonic cleaning system with an aqueous cleaning medium specifically designed for ultrasonic cleaners.
The problem is that after cleaning there is a white discoloration on the soldered areas. What could be the cause?
|Expert Panel Responses|
Without actually seeing the discoloration, it' difficult to tell if it's an oxidation problem possibly caused by a component of the cleaning agent or a true white residue issue. If it is a white residue problem, that may be even more difficult to pinpoint the cause as these may be rooted in the following:
We do not use the soldering materials/process that you do, so I can't add value to your question.
However, I want to comment on the cleaning process you describe.J-STD-001, 8.2.b prohibits ultrasonic cleaning of assemblies unless you have documentation (available for review) that " ...the use of ultrasonics does not damage the mechanical or electrical performance of the product or components being cleaned."
Obviously I don't know your design or the type of parts on the assembly, but I thought it should be pointed out that ultrasonic cleaning can damage quite a wide range of components.
NASA/Marshall Space Flight Center
Your residue could be one of two things:
If you find that the residues are residual flux, then you will certainly want to consult with both the paste manufacturer and the cleaning chemistry manufacturer to optimize the process.
Some no clean solder pastes are very difficult to clean. Typically this is due to making a paste pin probable so if this is what you are using, pin probable paste then you can either make a change in the paste or look at a different cleaner. I know Kyzen has developed a cleaner that works for these types of solder pastes. Feel free to contact me if you need specifics.
The white residue is uncleaned no clean flux residue. Ultrasonic cleaning does not clean well for most water soluble fluxes let alone for no clean flux residues. You should call me and we can discuss the risk of residues you are leaving in that no clean flux residue.
President/Senior Technical Consultant
Discoloration can be caused by several things. The first one I would check, is if the discoloration occurs across all solder joints.Is it a film or crystalline by-products of the partially removed flux? If it is a film only, it could be a surface reaction between the flux system and the SAC alloy.
Obviously if you notice crystalline residues usually white to off white in color it is flux residue that have not been removed. Doing ionic tests at this point is important to insure these forms of residues or reactions do not leave conductive residues behind. If ionic molecules are present, cleaning has not been effective.
It is also important to avoid extended times above liquidus during reflow, higher peaks and times above 217C will at times render the residues harder to remove. So lower temperatures, TAL or total area under this curve will make residues after soldering more easily cleaned.
The other concern would be effectiveness of the cleaning agent to remove completely the flux residues and the by-products of soldering. Cycle time, temperature and the concentration of the cleaning agent are important parameters to look at.
In reference to the thin whitish film, this is usually an oxidation reaction between the flux in the solder paste and the solder alloy,it is usually not an issue but mostly cosmetic. Ionic tests should be done to confirm this.
Senior Market Development Engineer
White residue left on the bump surface may be an indication of over-heat exposure during reflow. As circuit assemblies increase in density, more and smaller components are assembled on to a circuit card. Often times, this requires a longer soak time during the reflow soldering process.
In combination with higher lead-free reflow temperatures,the soils can thermally degrade with the potential to cross link and polymerize. These chemical reactions can render a cleanable solder paste more difficult to clean.
There are a number of other potential factors that may also be the root cause. One is that lead-free flux residues are more difficult to clean. They contain higher molecular weight materials to improve wetting and guard against thermal burn out.
If you have cleaned these residues with the existing process in the past, the issue is most like due to thermal heat exposure at reflow. If not, the issue may be a poorly match cleaning agent for the residue in question. Other factors to investigate are wash time, wash temperature, and bath maintenance.
I would check the compatibility of the flux to the cleaning solvent. The fluxes can be either rosin or resin based or water soluble based and this has to be defined prior to selecting the cleaning solvents to be used. Using solvents which don't remove all the residues will leave visual traces of the remaining flux films.
Many of the low-solid content fluxes are not made to be cleaned off and when subjected to cleaning solutions some of the residual films are partially removed and hence the light is reflected differently and appear as a whitish residues on the surfaces of the solder joints.
Since this is a paste application the solder joints and the periphery of the solder joints is the only place the flux residues are going to be visible and these most likely will be the areas that turn whitish in appearance.
Vice President, Technical Director
After many years of involvement in the PCB and precision cleaning industry the visible white residue issue by far has to be the most common and unpredictable anomaly associated in the fabrication and cleaning steps. Surface residues, such as flux residues, i.e. OA, Rosin or No-Clean and another typically defined as tin-salts which are insoluble in normal solvents,are generally considered a failure condition.
The chemistry and/or composition of the visible white residues are complex; the deposits themselves are of a number of types and origins, and their incidence is associated with both materials and process parameters attributed to before and after cleaning. Fluxing agents react with the tin and lead oxides on the solders' surface.
Doing what flux is designed to do it breaks down the oxides and allows the solder to flow and be joined. The oxides now are defined as metal salts thus effectively inert and water insoluble. Despite their water insolubility, under controlled processing conditions they will be washed away during the cleaning cycle as sufficient quantities of the fluxes soluble ingredients, in many cases the rosin component remains.
Enter the 21'st century and new paste formulas especially those termed"no clean" or low residue is the lower rosin % is now replaced by other active ingredients and thixotropic materials. Plasticizers and other "secret ingredients" sometimes listed on the MSDS. Halogen free solder paste formulas with SAC305 alloys are the most difficult of the new formulas to remove.
It is very important to match the cleaning agent to the flux residue.This is likely a prime factor in why you are seeing the white residue or it can be as simple as not being at the recommended in-use ratio of your cleaning agent. If it cleaned successfully before and now you are seeing white residue verify that the concentration percentage is correct. Also not defined is your wash temperature or time of wash.
These details are also important to confirm. If you wish to discuss this more do not hesitate to contact me.
Technical Expert Sales Support
I can suggest two areas of investigation, it is common for lead-free materials to react with alcohol. Perhaps there is a de minimus long-chain alcohol content in the cleaning material that is causing the chemical reaction with the flux that subsequently cannot be removed by the cleaning fluid.
Two tests suggest themselves:
White discoloration means a lot of possibilities... Could be partially cleaned flux residues, or some interaction with joint etc. A photo would help us a lot to identify the issue.I am suspecting the concentration could be low, or the temperature may not beat appropriate level.
Additionally we need to know the wash exposure time and the orientation of the boards inside the ultrasonic tank with respect to location of transducers. If you would like, contact us directly so that we can discuss the details and offer specific recommendations.
Application Technology Manager
A cleaning solvent that is chemically matched to the flux being used is required for satisfactory cleaning. It is also recommended that a cleanliness test of some type is performed as a process control to validate that the cleaning system is meeting the requirements of the contract. If the soldering process for each CCA requires more than an hour to complete, then in-process cleaning should be used pending completion of the soldering and the final cleaning. In your case this is the ultrasonic cleaner (US).
There is nothing wrong with using an ultrasonic cleaning system, as long as you understand its capabilities, and more importantly, its limitations. Be sure you are using a system designed for cleaning of electronics, not mechanical parts. The correct systems for electronics use a random sweeping oscillation to prevent cavitation in the cleaning fluid. Cavitation can totally destroy electronic component wire bonds and die bonds. I recommend Crest http://www.crest-ultrasonics.com/.I have no monetary interest, just a lot of really good experience with them.
Spray-in-air wash systems typically use some type of air knife after the final clean rinse to remove at least most of the rinse water from the CCA. A standard bench-top US system does not provide either a final rinse or a blow-off function. What this means is that the suspended flux solids(and other contaminants) are simply suspended in the wash solvent, be it water,IPA, or some other semi-aqueous solution. If the wash or rinse solution is allowed to simply evaporate, then the reaction products and any suspended solids are simply left behind on the surface of the product, and that is typically the white residue you see.
My recommendation would be to:
Is it residue? Take a solder sample from the same lot (do not print)and run through oven and then place in ultrasonic cleaner for similar time as your problem board(s).Robb Spoerri, USA
"Residue" present? If yes, look at condition of LPI mask cure. No? Print with paste, reflow and place in cleaner. Residue? Review what appears to be a solvent for the residue, do ionic contaminant testing, and investigate compatibility of cleaner chemistry with paste as recommended by others.
More than likely the Waxes as the Rosins will be normally very soluble portion of flux. The Paste manufacturers use Waxes for tack life and they are extremely difficult to remove if left on any longer than 48 hours after reflow. The Wax is insoluble in most normal Machine chemistries once aged any longer than 48 hours but can be cleaned using hand aerosol cleaners/defluxers due to the more active solvents used in aerosols.
For best results get them cleaned in the tank at around 60C the hotter the better, with a higher Ph solution or the US or FA+ works well from Zestron in Ultrasonics if done quickly.
Technical Sales Manager
BLT Circuit Services Ltd
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